全文获取类型
收费全文 | 691篇 |
免费 | 28篇 |
国内免费 | 2篇 |
专业分类
化学 | 515篇 |
晶体学 | 2篇 |
力学 | 26篇 |
数学 | 100篇 |
物理学 | 78篇 |
出版年
2023年 | 4篇 |
2021年 | 4篇 |
2020年 | 11篇 |
2019年 | 13篇 |
2018年 | 7篇 |
2017年 | 4篇 |
2016年 | 15篇 |
2015年 | 23篇 |
2014年 | 18篇 |
2013年 | 38篇 |
2012年 | 58篇 |
2011年 | 57篇 |
2010年 | 43篇 |
2009年 | 40篇 |
2008年 | 43篇 |
2007年 | 40篇 |
2006年 | 43篇 |
2005年 | 40篇 |
2004年 | 36篇 |
2003年 | 18篇 |
2002年 | 20篇 |
2001年 | 6篇 |
2000年 | 7篇 |
1999年 | 8篇 |
1998年 | 6篇 |
1997年 | 12篇 |
1996年 | 12篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1989年 | 6篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1982年 | 2篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1971年 | 3篇 |
1925年 | 2篇 |
1920年 | 2篇 |
1910年 | 2篇 |
1901年 | 3篇 |
1892年 | 3篇 |
1891年 | 2篇 |
排序方式: 共有721条查询结果,搜索用时 31 毫秒
61.
62.
Andrew C. Benniston Dr. Graeme Copley Dr. Helge Lemmetyinen Dr. Nikolai V. Tkachenko Dr. 《Chemphyschem》2010,11(8):1685-1692
Two series of geometrically‐related dyads are discussed based on the difluoroborondipyrromethene (Bodipy) unit, and incorporating covalently attached hydroquinone/quinone groups. These units are anchored directly, or via a phenylene spacer, to the Bodipy core at the meso position in one series ( BD‐MHQ , BD‐MQ , BD‐MPHQ , BD‐MPQ ), but for the second series the attachment site is the 2‐position ( BD‐SHQ , BD‐SQ , BD‐SPHQ , BD‐SPQ ). The compounds show various levels of fluorescence depending on the oxidation state of the appended group and the substitution pattern. In non‐polar solvents such as toluene, diethyl ether and dichlorobenzene, the S1 state deactivation of the Bodipy unit in BD‐SPQ and BD‐MPQ is dominated by 1, 3exciplex formation, which has not been reported for Bodipy derivatives so far. In the latter molecule, the decay of the exciplex is divided between population of the Bodipy triplet state (13 %–21 %) and ground state reformation. This partitioning is not seen for the side‐on substituted derivative, BD‐SPQ , and only ground state reformation is observed following decay of the exciplex. This difference in behavior is explained by the radical‐pair inter‐system‐crossing mechanism, which more effectively operates in BD‐MPQ because of the orthogonality of the donor‐acceptor units. In the more polar solvent CH3CN all the quinone derivatives show fast formation of the charge‐separated state (kCS) followed by slower charge recombination (kCR). The ratio kCS/kCR≤80. 相似文献
63.
Kirill Nikitin Helge Müller-Bunz Gérard Jaouen Michael J. McGlinchey 《Journal of organometallic chemistry》2010,695(4):595-608
The McMurry coupling of (tetraphenylcyclobutadiene)cobalt(cyclopentadienyl) ketones, (C4Ph4)Co[C5H4C(O)R], where R = Me, 3a, or Et, 3b, with a range of substituted benzophenones furnished a series of cobaltifens, organometallic analogues of tamoxifen whereby a phenyl ring has been replaced by an organo-cobalt sandwich moiety. These systems of the general formula (η4-C4Ph4)Co[η5-C5H4C(R)C(Ar)Ar′], where R = Me or Et, and Ar = Ar′ = p-C6H4X where X is OH, 2a and 2b, OMe, 2c and 2d, OBn, 2e and 2f, or O(CH2)2NMe2, 12a and 12b, and where Ar = C6H4OH and Ar′ = C6H4O(CH2)2NMe2, 2g and 2h, have been characterised by NMR spectroscopy and/or X-ray crystallography. The effect of 2a and 2b, 2g and 2h, and 12a and 12b on the growth of MCF-7 (hormone-dependent) and MDA-MB-231 (hormone-independent breast cancer cells) was studied. The dihydroxycobaltifens 2a and 2b exhibit a strong estrogenic effect on MCF-7 cells while the aminoalkyl-hydroxycobaltifens, 2g and 2h, were found to be only slightly cytotoxic on MDA-MB-231 cells (IC50 = 27.5 and 17 μM); surprisingly, however, the bis-(dimethylaminoethoxy)cobaltifens, 12a and 12b were shown to be highly cytotoxic towards both cell lines (IC50 = 3.8 and 2.5 μM). 相似文献
64.
Sebastian Gatz Helge Hannebauer Rene Hesse Florian Werner Arne Schmidt Thorsten Dullweber Jan Schmidt Karsten Bothe Rolf Brendel 《固体物理学:研究快报》2011,5(4):147-149
We demonstrate industrially feasible large‐area solar cells with passivated homogeneous emitter and rear achieving energy conversion efficiencies of up to 19.4% on 125 × 125 mm2 p‐type 2–3 Ω cm boron‐doped Czochralski silicon wafers. Front and rear metal contacts are fabricated by screen‐printing of silver and aluminum paste and firing in a conventional belt furnace. We implement two different dielectric rear surface passivation stacks: (i) a thermally grown silicon dioxide/silicon nitride stack and (ii) an atomic‐layer‐deposited aluminum oxide/silicon nitride stack. The dielectrics at the rear result in a decreased surface recombination velocity of Srear = 70 cm/s and 80 cm/s, and an increased internal IR reflectance of up to 91% corresponding to an improved Jsc of up to 38.9 mA/cm2 and Voc of up to 664 mV. We observe an increase in cell efficiency of 0.8% absolute for the cells compared to 18.6% efficient reference solar cells featuring a full‐area aluminum back surface field. To our knowledge, the energy conversion efficiency of 19.4% is the best value reported so far for large area screen‐printed solar cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
65.
Ville H. Pettersson Helge A. Tverberg Patric R. J. Östergård 《Discrete and Computational Geometry》2014,51(3):722-728
In 1911, Toeplitz made a conjecture asserting that every Jordan curve in $\mathbb{R}^{2}$ contains four points forming the corners of a square. Here Conjecture C is presented, which states that the side length of the largest square on a closed curve that consists of edges of an n×n grid is at least $1/\sqrt{2}$ times the side length of the largest axis-aligned square contained inside the curve. Conjecture C implies Toeplitz’ conjecture and is verified computationally for n≤13. 相似文献
66.
67.
68.
Christopher Sørum Nebojša Simić Eirik Sundby Bård Helge Hoff 《Magnetic resonance in chemistry : MRC》2010,48(3):244-248
Chemical shift assignment of seven N‐substituted 6‐(4‐methoxyphenyl)‐7H‐pyrrolo[2, 3‐d]pyrimidin‐4‐amines, six of which are fluorinated, have been performed based on 1H, 13C, 19F, and 2D COSY, HMBC and HSQC experiments. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
69.
Helge Tverberg 《Discrete and Computational Geometry》1989,4(1):191-203
In 1958 B. Grünbaum made a conjecture concerning families of disjoint translates of a compact convex set in the plane: if such a family consists of at least five sets, and if any five of these sets are met by a common line, then some line meets all sets of the family. This paper gives a proof of the conjecture. 相似文献